Such measurement transmitters are used for converting a process-related physical variable into an electrical variable, and are hence arranged as local, process-level devices in the field environment.
Known measurement transmitters are connected to a central device by means of a connecting cable, via which each measurement transmitter is supplied with electrical power for its operation and transfers data with the central device. When measurement transmitters are connected in the hazardous area of a process plant, electrical equipment must meet specific requirements in order to exclude the possibility of an accidental explosion. When installing and commissioning electrical installations and equipment and when maintaining electrical installations and equipment in process plants, which are located in a potentially explosive atmosphere because of their specific function, relevant legal regulations must be observed, such as the “Verordnung über elektrische Anlagen in explosionsgefährdeten Bereichen-ElexV” (German regulation on electrical equipment in hazardous areas) and the EN 50 014 series of European standards on explosion protection.
U.S. Pat. No. 6,457,367 B1 and the instruction manual “Contrans P Messumformer ARK 200 für Differenzdruck, Durchfluss, Füllstand” (Contrans P Measurement transmitter ARK 200 for differential pressure, flow rate and fluid level”) from Schoppe & Faeser GmbH, document number 42/15-190-3, disclose measurement transmitters whose enclosure essentially comprises two mutually separate containers, where the transducer for converting the process-related, physical variable into an appropriate electrical variable is housed in the one container, and an electronic circuit containing at least circuitry for measurement-signal processing and communication is arranged in the other container.
The two containers have complimentary screw threads to each other, and are screwed together directly. Electrical wires for connecting the transducer to the electronic circuit are fed through the screw connection. The electrical wires are part of a multi-core power-transmission and data-transmission cable.
It is known to have a fixed connection to the cable at one end and to have a plug-in design at the other end. If the cable is plugged in before screwing the containers together, the cable is twisted by the number of thread turns as they are screwed together, and then held in the twisted state. Alternatively, the cable can be wound up in an open reel sleeve in the manner shown in European patent EP 489 848 B1. In this case, there is the risk of damaging the cable or its connections at the connector or at the connection points at the opposite end of the cable from the connector. The number of thread turns depends on the type of thread and the permitted tolerances, taking into account the legal regulations cited in the introduction, where a minimum ratio of gap length to gap width must be maintained. In particular for the “flameproof enclosure” degree of protection, the cable needs to be twisted a plurality of times because a larger number of thread turns are required. This design is fault-prone, complex and difficult to handle.
The alternative is to plug in after screwing the modules together. This means it is necessary to ensure that the cable is not squashed when screwing the modules together. In addition, the end of the cable must be connected to the analysis electronics in a very confined space, and the electronics compartment must be opened.